The proliferation of cordless-communication devices has placed a severe strain on spectrum availability because most of the available "low-frequency" spectrum (i.e., spectrum below 900 MHz) has already been assigned. With the increasing demand for better communication systems, designers are forced to design and build systems at higher frequencies. However, with higher frequency designs come new problems, such as the stability and accuracy of a reference oscillator.
A block diagram of a typical dual conversion receiver is shown in FIG. 1, the operation and components of which are well known to those skilled in the art. This figure is referenced for delineating some of the obstacles that need to be addressed in a higher frequency system. Typically, the bandwidth of the intermediate frequency (IF) filter(s) 112, 118, 122 is small compared to the high frequency received signal. Therefore, to maintain proper operation, the requirements of the first local oscillator 108 become extreme in terms of temperature coefficient and frequency stability. For example, if the bandwidth of the IF filter 112 is 12 kHz, and the one-sided signal deviation is 5 kHz, then it is desirable to keep the frequency drift to less than plus-or-minus 2 kHz to prevent the signal from drifting out of the IF passband. For example, when the first injection signal is 855 MHz the signal must be held constant within a plus-or-minus 2 kHz, which results in approximately plus-or-minus 2.5 parts per million (ppm) crystal stability.
While such high frequency crystals are obtainable, the prices are often prohibitively high and require temperature compensation circuits to control the stability of the total oscillator circuit.
An alternative to using a high stability reference crystal is an Automatic Frequency Control (AFC) scheme. Conventional AFC schemes typically require a continuous wave (CW) signal to lock onto the carrier. Also, AFC circuits utilize a feedback control circuit that must be carefully designed to achieve loop stability over the operating temperature range and which also must withstand interference from spurious signals. Also, because conventional AFCs are coupled to the demodulator output, their operation is non-linear over temperature changes. Thus, a special temperature compensation circuit is generally required in a linear AFC circuit. This further sets a fundamental limit on the accuracy of any linear AFC. Using an AFC in this way is expensive, especially in low cost electronic devices such as selective call receivers. As a result, an AFC system has not been a cost effective solution for selective call receivers.
Thus, what is needed is an economical AFC scheme that increases the overall stability of a selective call receiver without the requirement of a high stability reference crystal oscillator.